1. Field
Embodiments of the present invention relate to image sensors, and in particular, to filters for image sensors.
2. Discussion of Related Art
Conventional image sensors may be fabricated from a complementary metal oxide semiconductor (CMOS) technology or from charge coupled device (CCD) technology. CMOS image sensors include an array of pixels disposed on a substrate, each pixel including a photosensitive element, such as a photodiode, for example, a filter disposed on the photosensitive element, and a micro-lens disposed on the filter. The photosensitive element and a floating diffusion region may be coupled together when an associated transfer transistor is turned on, to transfer the charge from the photosensitive element to the floating diffusion. There may be one or more layers of metal, polysilicon, diffusions, etc., disposed on the substrate as well.
The typical image sensor operates as follows. Light is incident on the micro-lens. The micro-lens focuses the light onto the photosensitive element through the filter. The photosensitive element converts the light into an electrical signal proportional to the intensity of the incident light. The electrical signal may be coupled to amplification and readout circuitry such as a CMOS transistor to generate an image based on the captured light.
Conventional image sensors suffer from some limitations. In image sensors that use front side illumination (FSI), the layers of metal, polysilicon, diffusions, etc., may be disposed between the micro-lenses and the photosensitive elements. During fabrication of image sensors that use FSI technology, a channel must therefore be created for light to travel from the micro-lens to the photosensitive element in an effort to avoid the metal, polysilicon, diffusions, etc.
One solution is to use back side illumination (BSI). In image sensors that use BSI, the layers of metal, polysilicon, diffusions, etc., are on one side of the substrate and the micro-lenses are on the other side of the substrate. Thus, there is no need to create a path to the photosensitive element that avoids the metal, polysilicon, diffusions, etc. Light is incident on the micro-lens and there is a direct path from the micro-lens and filter to the photosensitive element.
BSI image sensors suffer from limitations as well. For example, as the pixel size of BSI image sensors becomes smaller, it may be difficult for the micro-lens to focus incident light onto the photosensitive element. As a result, there can be crosstalk among the pixels. Crosstalk creates undesirable noise in the image sensor.